DIN rail mount for OpenFFBoard USB interface

This is a DIN rail mount for the OpenFFBoard USB interface. The holes are modelled at 2.5mm so they can be tapped for M3 screws, but you could open up the back side to fit short threaded inserts. Two of the holes in the board are too close to pins to be able to make the posts any bigger without interfering with the use of those pins (SPI 2 gnd and GPIO 6 and 8), so there's not enough meat to install inserts on the front side. If you're not using those pins, you can make the posts larger and install threaded inserts on the front side.